![\"Himawari-8](\"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-content\/uploads\/sites\/5\/2019\/02\/190226_himawari8_waterVapor_Pola_anim.gif\")
<\/a>Himawari-8 Low-level (7.3 \u00b5m),<\/em> Mid-level (6.9 \u00b5m)<\/em> and Upper-level (6.2 \u00b5m)<\/em> Water Vapor images [click to play animation | MP4<\/strong><\/a>]<\/p><\/div>Himawari-8 Low-level (7.3 \u00b5m), Mid-level (6.9 \u00b5m) and Upper-level (6.2 \u00b5m) Water Vapor images (above)<\/strong><\/em> revealed an interesting standing wave west\/northwest of Tropical Cyclone Pola in the South Pacific Ocean on 26 February 2019. The long-lived wave first became apparent just before 0800 UTC, and persisted until about 2330 UTC.<\/p>\n The standing wave feature was also apparent in Himawari-8 “Clean” Infrared Window (10.4 \u00b5m) images (below)<\/strong><\/em>. The abrupt warming of cloud-top infrared brightness temperatures associated with the wave suggests that subsidence was lowering the cloud height. Also note the very cold cloud-top temperatures of -90\u00baC and colder (yellow pixels embedded within the darker purple enhancement)<\/em> — this was colder than the tropopause temperature on 12 UTC rawinsonde data from both Nadi, Fiji (NFFN<\/strong><\/a>) to the southwest and Pago Pago, American Samoa (NSTU<\/strong><\/a>) to the northeast (the wave feature was located closer to the Nadi sounding).<\/p>\n Himawari-8 “Clean” Infrared Window (10.4 \u00b5m)<\/em> images [click to play animation | MP4<\/strong><\/a>]<\/p><\/div>Consecutive VIIRS Infrared Window (11.45 \u00b5m) images from Suomi NPP and NOAA-20, as viewed using RealEarth<\/strong><\/a> (below)<\/strong><\/em> showed a definitive bore-like structure with the wave, especially along the northern end.<\/p>\n VIIRS Infrared Window (11.45 \u00b5m)<\/em> images from NOAA-20 and Suomi NPP [click to enlarge]<\/p><\/div>Himawari-8 “Red” Visible (0.64 \u00b5m) images (below)<\/em><\/strong> showed the feature during daylight hours — a distinct shadow was being cast during local sunrise, which indicated a sharp drop-off in cloud height from east to west along the wave.<\/p>\n Himawari-8 “Red” Visible (0.64 \u00b5m)<\/em> images [click to play animation | MP4<\/strong><\/a>]<\/p><\/div>A HWRF-P model sounding for the latitude\/longitude point 15.42\u00baS\/179.75\u00baW valid at 18 UTC (source<\/strong><\/a>) showed directional wind shear at the 450 hPa pressure level — such a wind shear could have acted to initiate a horizontal roll circulation, creating a narrow zone of cloud-eroding subsidence. In addition, a sharp change in wind direction was seen above 150 hPa on the Paga Pago sounding<\/strong><\/a> — and the Nadi sounding<\/strong><\/a> showed speed shear with height — which also could have induced a horizontal roll circulation within the upper troposphere.<\/p>\n HWRF-P model sounding for the location 15.42\u00baS\/179.75\u00baW at 18 UTC [click to enlarge]<\/p><\/div>An interesting phenomenon was the apparent “shedding” of high-altitude cloud material from the higher\/colder cloud canopy of Pola immediately east of the wave feature, as seen in Himawari-8 Shortwave Infrared (3.9 \u00b5m) images (below)<\/strong><\/em>. The westward direction and velocity of this cloud material motion had good agreement with GFS model winds at 150 hPa. Note that this shed cloud material appeared warmer (darker gray)<\/em> in the 3.9 \u00b5m imagery — the shearing of cirrus cloud may have acted to fracture the ice crystals, making them smaller in size and therefore more efficient reflectors of incoming solar radiation.<\/p>\n Himawari-8 Shortwave Infrared (3.9 \u00b5m)<\/em> images, with plots of GFS 150 hPa winds [click to play animation | MP4<\/strong><\/a>]<\/p><\/div>A toggle between GOES-17 (GOES-West)<\/em> Infrared and Water Vapor images from the CIMSS Tropical Cyclones<\/strong><\/a> site (below)<\/strong><\/em> showed that the feature was aligned with a couplet of low-level convergence and upper-level divergence at 15 UTC — such an environment could also support a vertically-propagating gravity wave.<\/p>\n<\/a><\/a><\/a><\/a><\/a>